As the National Guard fanned out across the city yesterday, NYPD brass held a top-level meeting to discuss ways to combat a potential biochemical or suicide attack.

Although the specifics of the 31/2-hour meeting were kept under wraps, the NYPD said the most likely target of such deadly warfare would be the subway system, and the key stations would include Penn Station, Grand Central and the Atlantic Avenue complex in Brooklyn.

Cops said their “Heightened-Awareness Plan” dealt specifically with the transit system.

NYPD captains and other top brass received a briefing from the transit bureaus in Manhattan, Brooklyn, Queens and The Bronx, all of which did a threat assessment of their stations, looking for vulnerability to a nuclear, biological, chemical or suicide-bomber attack.

Cops studied evacuation plans and how to deal with mass hysteria.

The NYPD is in the process of ordering more gas masks – enough for every officer, sources said. In addition, they are looking to purchase special biohazard suits for the units that will first respond to the scene.

Meanwhile, new security measures were put in place at the city’s federal courthouse at 500 Pearl St. – including barring non-employees from using its front entrance.

The public may enter through a side entrance only, authorities said.

Chief Judge Michael Mukasey’s notice said all bags, parcels and containers will be checked, along with identification.

“Those wearing bulky clothing may be asked to open and permit examination of such clothing,” he wrote.

While the city’s safety measures were tightened in some areas, they appeared to be relaxed in others.

Cops weren’t seen checking truckers’ cargo before they crossed into Manhattan at the 59th Street Bridge, at least during morning rush hour.

At the entrance to the Midtown Tunnel also during the morning rush, there were six guardsmen directing traffic, as cops pulled over vans and trucks, opening their doors to check for potential explosives.

Some guardsmen did inspect the ID of bus drivers going into the tunnel.

At Grand Central Terminal, commuters said camouflage-wearing soldiers were evident during rush hour. The troops also were at Penn Station.

But even with the increased National Guard presence, some commuters said the Big Apple’s most vulnerable areas didn’t seem any safer.

“If the city thinks we need them, that’s great. But it doesn’t make me feel more safe and secure,” said Daniel Strachman, 30, of Manhattan.

For others, it gave a sense of security.

“Hopefully, nothing will happen,” said John Keane, 39. “But we need to be ready. Just to have them here is a good thing.”